Using the marketpress taxonomy

I was wondering if you may help me. MarketPress uses taxonomy (which i dont fully understand) to categorise its products separately to those belonging to the blog. I have edited line 18 on my theme (i know that was lightly irrelevant) to:
<?php $i = 0; $slider_query = new WP_Query("showposts=4&taxonomy=product_category&post_type=product&tag_ID=17"); ?>
It seems to display all the posts that are products. Rather than just those belonging to tag_ID=17 (which seems to be the unique identifier for each category)

Any help would be much appreciated